Cross-experiment user / exposure view

Feature request

Is your feature request related to a problem?

For power users with many (sometimes overlapping) experiments, there's no way to see, for a
person, all experiments they were exposed to and with which variants — nor to reason about interaction effects when a user sits in several experiments at once.

Describe the solution you'd like

• Person → experiments view/API. Given a person, list experiments + variants + exposure times.
• Overlap / interaction analysis (stretch) - Surface users in ≥2 concurrent experiments and flag potential interaction effects.

Additional context

Customer cares about where a user has been in all experiments or not, and wants to see behaviour based on this.

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start by mapping the existing experiment and exposure-data entry points relevant to a person, including where variants and exposure times are available. Define done as a person-level view or API listing all experiments, variants, and exposure times; overlap analysis is explicitly a stretch goal.